With respect to hitherto known lithographic printing plate precursors (hereinafter, also referred to as PS plates), since a step of removing the non-image area by dissolution (development processing) with strong alkali after imagewise exposure is performed, a post-processing step, for example, washing of the printing plate after the development processing with water, treatment of the printing plate after the development processing with a rinse solution containing a surfactant or treatment of the printing plate after the development processing with an oil-desensitizing solution containing gum arabic or a starch derivative, is necessary. The point that such additional wet treatments are indispensable is a large subject of investigation in hitherto known PS plates. Particularly, the consideration for global environment has become a great concern throughout the field of industry in recent years. In view of the consideration for global environment, a treatment with a developer closer to a neutral range and a reduced amount of waste liquid are subjects of further investigations. Further, it is desirable that the wet type post-processing is simplified.
From this viewpoint, as one method for eliminating the processing step, a method referred to as on-press development wherein an exposed lithographic printing plate precursor is mounted on a cylinder of a printing machine and the non-image area of the lithographic printing plate precursor is removed by supplying dampening water and ink while rotating the cylinder is known. Specifically, according to the method, the lithographic printing plate precursor is exposed and mounted on a printing machine as it is to complete development processing in a conventional process of printing (see, for example, JP-A-2005-125749 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). Also, a method is known wherein development is performed using a developer having a pH range lower than a developer used in hitherto known alkali development and after the development step, a post-water washing step and an oil-desensitizing treatment step (gum solution treatment step) are not conducted (see, for example, EP-A-1751625 and EP-A-1868036).
In such a lithographic printing plate precursor of a simple processing type, a support having a surface of high hydrophilicity is used in order to make possible development with a developer having pH lower than a hitherto known developer or dampening water (ordinarily nearly neutral) on a printing machine and as a result, the image area is apt to be removed from the support by dampening water during printing so that sufficient printing durability can not be obtained. On the contrary, when the surface of support renders hydrophobic, ink also adheres on the non-image area during printing to cause printing stain. Thus, it is extremely difficult to achieve a good compatibility between the printing durability and the stain resistance and further improvements are desired.
In view of the above problems, in JP-A-2006-239860, a lithographic printing plate precursor capable of forming an image without conducting alkali development and comprising a hydrophilic support having thereon a photopolymerizable layer sensitive to laser, wherein a copolymer which contains (a1) a repeating unit having at least one ethylenically unsaturated bond and (a2) a repeating unit having at least one functional group capable of interacting with a surface of the support and which is obtained by addition of isocyanate compound having a carbon-carbon unsaturated bond is contained in the photopolymerizable layer or other layer is proposed and it is described that a lithographic printing plate which is excellent in not only stain resistance but also printing durability is obtained.
Also, in JP-A-2008-213177, a lithographic printing plate precursor comprising a support having thereon a hydrophilic layer composed of a hydrophilic polymer which contains at least one of a reactive group capable of directly chemically bonding with a surface of the support and a reactive group capable of chemically bonding with a surface of the support through a crosslinked structure and a partial structure having a positive charge and a negative charge and which is chemically boned to the surface of the support and an image-forming layer in this order is proposed and it is described that a lithographic printing plate which is excellent in hydrophilicity of the non-image area and its sustention and also excellent in adhesion property between the image area and the support is obtained.
However, with respect to the lithographic printing plate precursor proposed in JP-A-2006-239860, the stain resistance of the lithographic printing plate prepared is insufficient and further a problem arises in that stain occurs when printing is carried out after the passage of time from the preparation of lithographic printing plate, that is, the stain resistance after storage is insufficient.
Also, with respect to the lithographic printing plate precursor proposed in JP-A-2008-213177, sufficient results in both the stain resistance without storage and the stain resistance after storage are not obtained. Further, as for the hydrophilic polymer described in JP-A-2008-213177, there is a restriction in the producing condition, for example, it is necessary to use a nonaqueous solvent at the polymerization reaction in order to introduce the reactive group capable of chemically bonding with a surface of the support through a crosslinked structure into the polymer and environmental load is large.